The math is neutral if you only look at that very transaction, you're right. However in the bigger picture (by looking at the entire blockchain), you might determine that all other inputs have been spent with no mixin, therefore revealing the last input that must be the correct one. There is some quite fancy way to infer information probabilistically too, all developed in the latest Monero Research Lab article: https://lab.getmonero.org/pubs/MRL-0004.pdf
So nobody was reinforcing FUD, it was simply thought that apparently chainradar was trying to infer (heuristically) the correct input. Though on the basic math level, again you're right, there's no way to tell, and that was never questionned of course.